period when winter in the Northern hemisphere coincides with the earth's position in its orbit farthest from the sun, the mean temperature of the Northern hemisphere will be considerably lower than when the reverse conditions prevail—that is, when winter in the north coincides with the earth's position in its orbit nearest to the sun, as at present. Moreover, this change of position of the earth in its orbit would likely result in a change in direction of the trade winds, the air currents being mainly caused by the expansive action of heat creating a vacuum into which the air rushes from colder areas; therefore, as the Southern hemisphere would become the warmer, the prevailing winds would be southward, thus changing the direction of ocean currents, like the Gulf Stream, to the southward, this being a secondary cause resulting from the first and further intensifying the cold.
This hypothesis is supported by the fact that at present winter in the Southern hemisphere coincides with the earth's position in its orbit farthest from the sun, and the ice around the south pole extends much farther toward the equator than that around the north pole, the antarctic ice extending as far as the sixty-seventh degree of latitude. The antarctic summers are also said to be more humid, cold, and chilly than the arctic summers.
But the earth's orbit is not always the same; there have been periods when the orbit described resulted in a considerably greater difference than three million miles, and it is thought that if this ordinary eccentricity of the orbit is insufficient to account for the Glacial epoch, the periods of greater eccentricity would.
This hypothesis seems to explain rather overmuch, for if true, it accounts not only for the Glacial period, but for many glacial periods in the past, for, other conditions remaining the same, there would result a Glacial period for every period in the past that the earth held the proper relative position in its orbit; and if true, there should be geological evidence to sustain it, which there does not appear to be, for such evidence of earlier glaciers as the rocks of past geological ages exhibit would seem to indicate local glaciers, not any widespread glacial action; but it would be rash to maintain that the other conditions remained the same; geology can not be said to show that they did.
Still another hypothesis is that the Isthmus of Panama was in glacial times submerged, thus allowing the Gulf Stream to flow into the Pacific Ocean and thence north; also that the northern coast of British America was more elevated than at present. There is geological evidence to sustain both these propositions, but not conclusively. It is obvious that under these conditions the eastern part of the northern coast would be much colder, being more elevated and having lost the heat emanating from the Gulf Stream, while the western part would be warmer, having